2000 GT 3.4 P304 Misfire

[shyguy5885]

2000 Grand Am GT with 3.4 and 150,000 miles. Car keeps throwing code P304 - cylinder 4 misfire. The main issue is the light and needing to pass emissions test. It has just recently had new plugs, wires, and injector #4.

The fuel pressure regulator was replaced about 3 years ago because it was leaking fuel, enough to smell while driving the car with windows rolled down. Shortly after the fpr was replaced, the acceleration problem seems to have started, either by coincidence, or improper installation. The light/error has been occurring for quite some time, about the past year or two. About a year ago, the car was also throwing a random misfire code. Im not positive if both codes were being thrown at the same time, and when exactly the P304 started. The crank (or cam) sensor was replaced and helped with the issue for a short while (removed codes/light for maybe a few months?) Before the sensor was replaced, the car would stumble at very slow speeds, 0-10 mph. Also replaced Idle Air Control valve last year. The car would sometimes surge on semi warm starts (not cold, not 190-200) between 500-1000 rpms. The surge would be worse when I had the blower running or the rear defrost on, down to 200 rpms at times.

Currently, the car runs perfectly fine for me, except when trying to accelerate on highway ramps. Its firing somewhat when you step on the pedal, but lags and takes longer than normal to accelerate up to speed for merging with traffic.

Had a mechanic remove newer Bosch 4 plugs and replace with standard AC Delco, kept Bosch wireset. He checked compression on cylinders 1, 4 and 6 (radiator side) and readings were 160 160 and 140. He concluded everything mechanical inside the engine should be ok so it had to be a bad/dirty injector. He advised me to use Lubro Moly injector cleaner and only fill with Shell V power. This seemed to work for a week or two, light went off and seemed to run a little bit better. But eventually, the light came back on before all the pcm checks would reset (after erasing code and emission data). Mechanic then replaced injector #4 and misfire was still occurring. The coil pack was also swapped out and still getting misfire. Mechanic also removed valve cover for a quick visual inspection but couldnt find any issue - he said he'd previously seen 3.1/3.4 with loose rocker arm bolts. His conclusion now is that the issue is more serious, and big money for repairs - burnt intake valve, cracked head or head gasket starting to go. He also drove the car after new injection and said it runs fine while driving, but the misfire seems to be mostly or only occurring at idle.

I'm just looking to pick your brains here. I cant afford to keep throwing money into things that should solve the problem and dont. I saw a previous post by xero (?) about the injector harness/wires being faulty and only working some of the time. Do you recall if the wire failure was between the injector and harness connection or between the master connection and the pcm? My ga has been pretty good to me so far, I dont want to lay him to rest yet haha. Thanks for any tips or ideas.

[GTP Dad]

First of all with compression readings like you have there are no mechanical problems that need attention at least ones that haven't already been checked. I suspect you have a bad connection at the injector. These wires are fragile and can get broken or semi-broken from rough handling or just from normal use. I would get a noid light from a local parts store and check the wiring to see if the injector is firing properly. This light will indicate if the problem is in the wiring harness or at the injector itself. After checking the wiring if you find no issues put some dielectric grease on the connector and reinsert it into the injector. This may be enough to eliminate the problem. Good Luck!

[Tech II]

Problem is, you can't check the injector harness with a noid light without removing the top of the Plenum.....what you can do is an ohm check at the injector bulk connector, while reading ohms, stick something under the plenum(that won't damage the wires), and see if the reading changes....

I'm sorry, but methinks we are involved with a parts changer here, rather than a diagnostician......

Also very confusing.....you say it happens under a load, he says it happens at Idle.....

He did a compression of 2-4-6......have seen situations where, even though P0304 comes up, sometimes it's the "mating" cyclinder, #1, that is the problem......are you losing any coolant?

Maybe the injector was the problem.......but what if he replaced it with an aftermarket injector, that doesn't spray the same as the OEM ones? Have seen this before.....

Even though the wires were replaced, could have a bad wire....would swap #4 with #2 or #6 to see if the misfire moved......if nothing changed, then would do the same with #1 on the backside......when doing this, should have a scan tool that reads misfire data on all cylinders, to see EXACTLY which cylinder is misfiring....

Was coil output ever checked with a spark tester?

[shyguy5885]

As a side note, I actually attempted to replace the injector myself. I got as far as the fuel rail and decided I should pay someone instead of breaking something or causing a fuel leak to develop. I can imagine why mechs want 3 hours to do the work, its a nightmare getting the upper plenum and everything else off the top.

If I feel the courage to rip everything down to the rail again, how exactly are the connectors removed from the injector? I saw a green clip but was unsure how to remove without completely ripping the wires out and opening a can of worms. Also, are there any tips or special tools required to remove the fuel line from inlet and return (under fpr)? Does it need teflon tape, or some sort of pipe putty when reassembling? I attempted to remove but again, the rail was starting to twist so I thought better to stop messing with it. I have a Haynes manual, but it does not explain any details.

Yes, I meant to say Cylinders 2 4 and 6. I apologize if my post was a bit confusing, it has been quite some time since my last posting and visit to the site. Wouldnt the mating cylinder be directly above, #3? Ive always had coolant issues with the car, I'm sure I'm not alone. I had lost at least a recovery tank's worth in about 3 weeks, since the mech first looked at the car. I cannot tell if it was an internal or external loss though the oil looks ok. Also, my heater bypass hose is bulged, so the coolant loss could have occurred there or there was some air stuck in the system which caused a boil over. I purchased the parts myself, injector is Gp sorenson. Mech made the same comment about it could be a bad part/brand. I also watched him use a noid light (touching stem of injector) and 2 4 and 6 lit and pulsed. If I do decide to swap out the injector 4, which brand is original, Delphi or AC Delco? I do not know if coil output was checked with spark tester. Could you explain the resistence check - which connections and what the values should be? I browsed through my Haynes manual to the chassis electrical section and I see that all injectors (except number 4) are on one master connection while #4 is on another. Can i add di electric grease to the pcm connections without causing harm? I do not have a scan tool to swap the plug wires, but could i check resistance for wire #4?

[Tech II]

The only way you can check injectors is with special tools......an ohm check, is too much of a static test(it is only valuable if one injector is way off in resistance from the others)......to really test an injector, it has to be powered....you can test the coils of the injector....and then you check the flow of the injector over a finite time...if either test fails, the injector is bad......but the specs for some aftermarket injectors can be off just enough to make the injector run a little richer or a little leaner than the other injectors.....if you can't replace an injector with an OEM one, and have to use after market, I try to talk the customer into using all new aftermarket, so there won't be any disparity in flow between injectors....

As for procedures, get a GM manual instead of a Haynes....

Mated cylinders are 1&4, 5&2, and 6&3.....

Need to find if the coolant leak is external or internal......could be lower intake gaskets, w/p, radiator, heater core, hoses, etc for an external leak, and a head gasket/head or lower intake for an internal one.....anytime you lose coolant, you create an air pocket which can cause an overheating/boil over condition....

Injector #4 is in the same harness as the rest of the injectors......In C130, it is labelled terminals "A" and "F".....should be around 12 ohms.....compare the reading to say, #1 injector, terminals "A" and "C" and also to #6 injector, terminals "A" and "H".....while reading "A" and "F", move the harness at the injector connector at #4, and back towards the main connecto, C130(remember, injectors are located on the opposite side of the cylinder)......if the OHM reading changes at all during movement, there is a wiring problem....

[rkvons]

Originally Posted by shyguy5885

If I do decide to swap out the injector 4, which brand is original, Delphi or AC Delco?

Delphi is the OEM manufacturer for your injectors.
Delphi FJ10066 Fuel Injector

[shyguy5885]

Thank you rkvons - I had assumed AC Delco was original but kept seeing Delphi appear on some sites saying it was OEM.

I did a brief check yesterday and resistance across coil pack was similar to others, about 6500 ohms. I checked the resistance at the injector harness and readings were about 14.5 ohms, same between A/C A/F & A/H. Today I will start removing the octopus of wires and remove the upper plenum. When doing voltage and ohm readings on the injector connection and injectors, what should values be? Also, I figured if I have to tear it apart again, it might be worth removing all injectors and sending them out for a Flow Test/Cleaning? Is this actually beneficial or is it a gimmick?

Thanks for the tips so far

[thephantom1492]

All fuel in the USA and canada must have a cleaner already. So unless you use old fuel or ain't in north america, then adding cleaning fluid is just wasting money.

Since the fuel does contain a cleaning agent, the injector shouln't be dirty.

That said, there is always a possibility of contamination due to whatever reason.

I doubt it's an injector issue, check the wiring.

[rkvons]

I have a car that was really running bad. Did not know what was causing it. Went to a junk yard and pulled six injectors off of similar cars (checked the part numbers to make sure they were correct), cleaned them all ahead of time and then swapped them out. Takes 1 - 2 hours. Car is purring like a kitten now. The injectors were about $7 apiece. I've also ordered cleaned injectors from an online place and that worked too. It is not a gimmick. Your injectors do get dirty over time and once they get dirty enough, you can't clean them with additives in your tank anymore. The additives will only keep them clean.

[shyguy5885]

Finally got a chance to get everything taken off. A few blisters and swollen fingers later, I got the injectors pulled out of the rail...holy heck are they in there tight. Will send the injectors out to have them flow tested and cleaned. While the rail was still on, I did a brief voltage check on 2 injectors (only two connectors I could remove, those stupid green tabs). Voltage was only 6.5 for both, but I figured it must be normal or just because all the other sensors were disconnected. Tech, you mentioned something in a previous post that has been stumping me - you said remember, injectors are located on the opposite side of the cylinder. Are you saying that from a bird's eye view, what would appear to be lower left injector on rail is not the injector that feeds Cylinder #2? I've also been throwing around the idea of replacing the LIM gasket while I'm waiting for the injectors. I read about a similar issue of someone having a cyl 5 misfire and it ended up being the intake gasket leaking coolant into the intake and combustion area of head. Original gaskets were replaced by GM dealer, but back in Feb05 at 57,000 miles. Will keep everybody posted.

[Tech II]

Should be battery voltage, not 6.5 volts....

Look at the lower intake, you have 6 air openings, and 6 injector holes......

The front cylinders are 2-4-6, the rear 1-3-5....

Looking at the front row of injector holes, you would think the left one would go to cylinder #2, but instead it goes to the opposite side, cyl #1.....in fact, if you look at the runners in the lower intake, they are numbered.....air(along with the fuel) enters on one side and travels across inside the lower intake to the opposite side...

[shyguy5885]

Tech, I think I'm understanding you but I want to confirm with a photo before I start getting really angry with the mechanic that replaced one injector. In the photo, circled in blue is the injector that feeds Cylinder #4 and in green feeds Cylinder #3?

BTW, I have since cleaned the surface dirt so it doesnt fall into the ports. Do I need to clean the top part of the injector ports with carb/throttle body cleaning before installing new injectors? Add a light coat of oil around the rim?

https://picasaweb.google.com/1139880...3/FuelInjector

[Tech II]

Sorry, my mistake......the injectors are NOT on the opposite sides, just the air inlet......if you look at your picture and the middle air inlet at the front side, see how the runner is labeled "3"? the air comes in, crosses over, and mixes with injector #3, which is your blue circle...the injector that is covered with the plastic is #1...

Sorry for the confusion....

[shyguy5885]

My injectors are on their way back - they checked out fine and didnt even need the cleaning according to flow tests. I sent them to Rich at Cruzin. He was honest enough to 1, save me money and 2, save his time wasted for cleaning them. While I'm waiting for them, I decided to tackle to LIM change. Is it safe to soak valve covers, intake and plenum in a bath of warm water and dawn dish detergent? Or if I opt to get a chemical, should I use general engine degreaser, throttle body cleaner, brake parts cleaner? Thanks guys, will keep you posted as I move things along.

[rkvons]

Originally Posted by shyguy5885

My injectors are on their way back - they checked out fine and didnt even need the cleaning according to flow tests. I sent them to Rich at Cruzin. He was honest enough to 1, save me money and 2, save his time wasted for cleaning them. While I'm waiting for them, I decided to tackle to LIM change. Is it safe to soak valve covers, intake and plenum in a bath of warm water and dawn dish detergent? Or if I opt to get a chemical, should I use general engine degreaser, throttle body cleaner, brake parts cleaner? Thanks guys, will keep you posted as I move things along.

I don't know about soaking them. Water and oil don't mix well. But a can of carb cleaner will make you smile. And I don't mean from inhaling the fumes.